The CWE-476 vulnerability is a null pointer dereference. In this case, the vulnerability occurs at line 7 because the `to_usb_interface` function does not check if the input is null before dereferencing it.

The root cause of this vulnerability is that the `hub->intfdev` is not checked for null before being passed to the `to_usb_interface` function. If `hub->intfdev` is null, the `to_usb_interface` function will dereference a null pointer, resulting in a null pointer dereference error.

To fix this vulnerability, a null check should be added before calling the `to_usb_interface` function. Here's an example of how the code could be modified to fix the vulnerability:

```c
static ssize_t disable_show(struct device *dev,
        struct usb_device *hdev = to_usb_device(dev->parent->parent);
        struct usb_hub *hub = usb_hub_to_struct_hub(hdev);
        struct usb_interface *intf;

        if (hub->intfdev == NULL) {
            // Handle the error, for example, return an error code
            return -EINVAL;
        }

        intf = to_usb_interface(hub->intfdev);
```

By adding a null check before calling the `to_usb_interface` function, we can prevent the null pointer dereference and fix the CWE-476 vulnerability.